Aromatic hydrocarbons, notably benzene, toluene, and xylenes are important industrial commodities used to produce numerous chemicals, fibers, plastics, and polymers, including styrene, phenol, aniline, polyester, and nylon. Typically, such aromatic hydrocarbons are produced from petroleum feedstocks using well-established refining or chemical processes. More recently, there is a growing interest in providing aromatic hydrocarbons from alternative resources, such as biomass, synthesis gas and natural gas.
One possible resource is plant biomass. Plant biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls in higher plants. Plant cell walls are divided into two sections, primary cell walls and secondary cell walls. The primary cell wall provides structure for expanding cells and is composed of three major polysaccharides (cellulose, pectin, and hemicellulose) and one group of glycoproteins. The secondary cell wall, which is produced after the cell has finished growing, also contains polysaccharides and is strengthened through polymeric lignin covalently cross-linked to hemicellulose.
The resulting composition of the biomass provides roughly 40-50% cellulose, 20-25% hemicellulose, and 25-35% lignin, by weight percent. Cellulose is typically the primary sugar source for bioconversion processes and includes high molecular weight polymers formed of tightly linked glucose monomers. Hemicellulose is generally considered a secondary sugar source and includes shorter polymers formed of various sugars. Lignin includes phenylpropanoic acid moieties polymerized in a complex three dimensional structure and is often viewed as a waste material or byproduct useful for other processes. Collectively, the components of cellulose, hemicellulose, and lignin are often referred to as oxygenated hydrocarbons.
Heterogeneous catalysts have shown great promise for converting biomass-derived oxygenated hydrocarbons into fuels and chemicals. A difficult challenge to overcome is obtaining high yields of select hydrocarbons while minimizing coke formation and catalyst deactivation.
Chen et al. developed the hydrogen to carbon effective (H:Ceff) ratio as a tool to assist in determining the suitability of oxygenated hydrocarbon feedstocks for catalytic conversion to hydrocarbons using zeolite catalysts (N. Y. Chen, J. T. F. Degnan and L. R. Koeing, Chem. Tech. 1986, 16, 506). The H:Ceff ratio is based on the amount of carbon, oxygen and hydrogen in the feed, and is calculated as follows:
                              H          ⁢                      :                    ⁢                      C            eff                          =                              H            -                          2              ⁢                                                          ⁢              O                                C                                    (                  equation          .                                          ⁢          1                )            where H represents the number of hydrogen atoms, O represents the number of oxygen atoms, and C represents the number of carbon atoms. Water and molecular hydrogen (diatomic hydrogen, H2) are excluded from the calculation. The H:Ceff ratio applies both to individual components and to mixtures of components, but is not valid for components which contain atoms other than carbon, hydrogen, and oxygen. For mixtures, the carbon, hydrogen, and oxygen atoms are summed over all components exclusive of water and molecular hydrogen. The term “hydrogen” refers to any hydrogen atom while the term “molecular hydrogen” is limited to diatomic hydrogen, H2.
Zhang et al. studied the impact of the H:Ceff ratio on the conversion of various biomass-derived oxygenated hydrocarbons to coke, olefins and aromatics using a ZSM-5 catalyst (Zhang et al., Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio, Energy Environ. Sci., 2011, 4, 2297). Zhang reported that biomass derived feedstocks having H:Ceff ratios of between 0 and 0.3 produced high levels of coke, making it non-economical to convert such feedstocks to aromatics and chemicals. However, by hydroprocessing the feedstock to add hydrogen, Zhang was able to produce aromatics and olefins using a ZSM-5 catalyst at yields 2 to 3 times higher than a process without hydrogenation. Specifically, Zhang reported that the aromatic and olefin yields increased from 12% to 24% and 15% to 56%, respectively, with increasing H:Ceff ratio. The ratio of olefins to aromatics also increased with increasing H:Ceff ratio, with the olefin yield higher than the yield of aromatics for all feedstocks. It was also reported that there is an inflection point at a H:Ceff ratio of 1.2, where the aromatic and olefin yield does not increase further, indicating that at most the yield of high value aromatic chemicals, such as benzene, toluene, and xylenes (BTX), may be limited to 24% when using zeolite catalysts according to the Zhang process.
In another study by Fuhse and Bandermann, the researchers studied the conversion of a number of different types of oxygenates over a ZSM-5 catalyst to aromatic hydrocarbons (Fuhse and Bandermann, Conversion of Organic Oxygen Compounds and their Mixtures on H-ZSM-5, Chem. Eng. Technol., 1987, 10, 323-329). The researchers reported oxygenates having H:Ceff ratios less than 1.6 cause the problem of coking, decreasing the catalyst's lifetime. The researchers also report that conversion of carboxylic acids and esters cannot be explained solely by the H:Ceff ratio because these types of reactants undergo the side reactions of decarbonylation, decarboxylation, and ester pyrolysis. For example, the researchers reported that the reaction of acetic acid yields only acetone and CO2. Moreover, when the researchers investigated mixtures, the researchers stated that the conversion of mixtures to products depends on the individual components, and that when oxygenates having a H:Ceff less than 1.6 are added to the mixture the yield of aromatic hydrocarbons decreases.
Because of the low hydrogen to carbon effective ratio of carboxylic acid feedstocks and the likelihood of side reactions, conventional wisdom teaches that these feedstocks are unsuitable for conversion to aromatic hydrocarbons in high yields. As a result, there exists a need for methods and systems to effectively and efficiently convert carboxylic acid feedstocks to aromatic hydrocarbons.